The physical and electronic properties of aluminum nitride (AlN) give it great potential for a wide variety of semiconductor applications. AlN has a wide energy bandgap (6.2 electron volts), high breakdown electric field and extremely high thermal conductivity. In fact, in Chow et. al Wide Bandgap Compound Semiconductors for Superior High Voltage Unipolar Power Devices (IEEE Transactions on Electron Devices, Vol. 41, No. 8, 1994) ranking all semiconductors materials, AlN is reported to have, excluding diamond, the highest figure of merit for unipolar power device performance.
In addition, the high thermal conductivity and high optical transmissivity (i.e., low optical density) of AlN make AlN an excellent candidate substrate material. Also, AlN is likely to be the optimum substrate for the growth of pseudo-binary inter metallic compounds such as Al.sub.0.8 In.sub.0.2 N which have extremely high figures of merit for semiconductor performance (up to 4,413,000 times silicon). Although AlN has extraordinary properties for a semiconductor material and has tremendous commercial potential, AlN based semiconductor devices have been limited by the unavailability of large, low defect AlN single crystals. In the most successful prior work, Slack and McNelly demonstrated a method for growing AlN single crystals via sublimation in AlN Single Crystals (Journal of Crystal Growth 42, 1977). However, the time required to grow a 12 mm by 4 mm crystal was approximately 150 hours. This growth rate is far too low to ever allow the commercial production of AlN single crystals.
AlN has been alloyed with silicon carbide (SiC) in monocrystalline thin films produced, for example, by liquid phase epitaxy. Polycrystalline AlN:SiC alloys have also been produced by isostatic pressing processes. However, bulk single crystalline (monocrystalline) alloys of AlN:SiC have not been commercially produced.
Certain monocrystalline AlN:SiC alloys have promise for use as substrate materials that have superior electronic properties to either AlN or SiC. Physical and electronic properties can be tailored to specific device applications by selecting specific AlN:SiC alloy composition. For example, certain AlN:SiC alloys are direct bandgap materials which is an important property for optoelectronic devices and other electronic device applications. Furthermore, AlN:SiC alloys have other desirable electronic properties such as high electron hole mobility, high electron break down field, high saturated electron drift velocity, high thermal conductivity and a wide energy bandgap. Accordingly, there is a need for apparatus and processes for growing bulk single crystalline alloys of AlN:SiC, particularly such single crystalline alloys that are tailored in their makeup to meet specific needs, for example, specific needs in the electronics industry.
Silicon carbide (SiC) is rarely found in nature. It has, however, been manufactured for more than eighty years, in crystalline form, for abrasive products. Silicon carbide crystals found in nature and in abrasive products are generally black and not translucent because they contain substantial levels of impurity atoms.
In the 1950's the Lely process was developed at General Electric Company by which silicon carbide was sublimed and randomly deposited to produce small, thin silicon carbide crystals that were used in early silicon carbide semiconductor device development.
Because of the theoretically quite favorable electronic properties of silicon carbide, significant development activities were initiated during the 1960's and 1970's with the objective of growing large (bulk) crystals of low impurity silicon carbide for use in the production of semiconductor devices. These efforts finally resulted in the commercial availability of relatively low impurity, translucent silicon carbide crystals. These silicon carbide crystals are fabricated and marketed as very thin green, amber or blue (175 .mu.m-400 .mu.m) slices useful for semiconductor devices.
Recently, it has been discovered that relatively low impurity, translucent, single crystal silicon carbide may be grown with a desired color and thereafter fashioned by faceting and polishing into synthetic gemstones. These gemstones have extraordinary hardness, toughness, chemical and thermal stability, and a high refractive index and dispersion that produce unparalleled brilliance. The single crystals from which the gemstones are produced have been grown by sublimation according to techniques of the type described in U.S. Pat. No. Re. 34,061.
Silicon carbide crystals can be grown in a wide range of colors (including green, blue, red, purple, yellow, amber and black) and shades within each color by the appropriate selection of dopants (e.g., nitrogen and aluminum) and by varying the net doping densities (concentrations). Because of its wide bandgap, undoped ("intrinsic") silicon carbide crystals in the hexagonal or rhombohedral forms are inherently colorless. Thus, silicon carbide crystals offer the potential to be faceted and polished into gemstones of many various appearances, including that of relatively colorless diamond.
Because of the emerging recognition of the value of aluminum nitride, silicon carbide and alloys thereof for electronics applications and otherwise, there has developed an acute need for a low cost, high output process for producing low a defect density, low impurity bulk single crystals of these materials.